Hazardous waste materials have been improperly deposited in thousands of sites all over the United States and, indeed, all over the world. Uncontrolled landfills have been used as convenient, but inadequate, disposal sites for industrially generated wastes while other sites have been contaminated by accidental spills of hazardous materials. There are many sites where hazardous materials were spilled prior to the discovery of the hazardous nature of the materials being handled. Many hazardous materials found at these sites are stable, do not undergo environmental degradation at reasonably fast rates, have high boiling points, are considered toxic at very low concentration levels, and bio-accumulate in various species of the food chain at concentrations higher than that found in the environment.
Complete reclamation or isolation of such sites is preferred but the cost associated with site disturbance by available methods has been considered prohibitive. The treatment of contaminated soil from such sites in an incinerator has not been a practical solution for several reasons including the high cost of excavation and incineration, shortage of incineration capacity, inadequate methods and capacity for ash disposal in the incinerators, and the hazards and risks associated with site disturbance and transportation. The treatment of uncontrolled landfills and spills would benefit from an in situ process that eliminates or alleviates these disadvantages and risks. Radio frequency (RF) heating applied in accordance with the present invention offers a viable in situ method for treatment of contaminated sites.
The term "RF" refers to frequencies used in wireless communication and represents a wide frequency range from 45 Hz to 10 gigahertz (GHz). However, the frequencies of interest for in situ soil heating lie principally between 0.5 to 45 MHz. For dry soils, in this frequency range, dipolar molecules absorb electromagnetic (EM) energy which is converted to heat due to dipole rotation and molecular vibration. This is known as dielectric heating. The absorption of EM energy and conversion to heat occurs throughout the volume of the material and is not dependent on the relatively slow process of thermal conduction. The amount of energy dissipated in the heated soil is proportional to the dielectric constant, the loss tangent, frequency and the square of the field strength of the applied electromagnetic energy. The penetration depth of the applied fields is inversely related to frequency and to the conductivity of the soil. Thus, for any given soil, frequency may be selected to provide the required penetration depth. Penetration of EM energy of a few to more than 50 meters can typically be achieved.
In situ heating of earth formations by high-frequency displacement currents (dielectric heating) is well known, particularly in the production of petroleum products such as shale oil. Alternatively, heating by conduction currents at relatively low frequencies is also possible, but such heating is limited to earth that remains conductive, generally requiring the presence of water and, hence, operating at relatively low temperatures below the boiling point of water or requiring maintenance of pressure. Conduction heating at very high temperatures for the immobilization of radioactive components in soil is shown in Brouns et al., U.S. Pat. No. 4,376,598, where conductive material was added to the soil to assure conduction, and the soil was heated to vitrification at temperatures as high as 1500.degree. C., whereat radioactive contaminants are fused with the silicates in the soil to form a glass or similar product which, upon cooling, forms a stable mass.
In situ heating of earth formations with RF for hydrocarbon production is shown in Bridges et al, U.S. Pat. No. Re. 30,738 and Kasevich et al U.S. Pat. No. 4,140,179. The former discloses the use of RF from a "tri-plate" line buried in the earth to heat a block of earth formations uniformly by displacement currents, leading to dielectric heating. The latter discloses radiating RF energy into the earth. In U.S. Pat. No. 4,670,634 a portion of the earth near the surface is decontaminated by selective heating with RF energy from a transmission line array to which the RF energy is bound. That is, there is substantially no radiation from the bound-wave fringing-field transmission line excitor.
Existing RF heating methods, such as the above-referenced fringe field or tri-plate methods, suffer from high impedance coupling, and high voltages must be used to achieve moderate heating rates. Consequently, typical treatment times may be on the order of months to achieve the desired temperature. This is especially the case when water, or a water-based solution, has been added to the soil to enhance the treatment. The electrical breakdown of air determines the theoretical upper limit of voltage which can be applied; however, lower practical limits may be set by RF generator and transmission component limitations and resistive losses.
Referring now to FIG. 1, the prior art bound-wave fringing fields applicator 10 (shown in the '634 patent) consists of a set of parallel electrodes 12, 14 located above the earth's surface 13 to which alternating voltages are applied (the same voltage between each adjacent pair, with alternating polarity). Lines 11 represent lines of displacement current expected where the deposit is relatively dry near the horizontal wire pairs 12, 14. In this method, little heating occurs directly under each electrode. Therefore, to make the heating more uniform, the power may be switched periodically to interspersed electrodes. Heating is dependent entirely upon the electric field established.
Referring now to FIG. 2, the "tri-plate" transmission line applicator 15 (U.S. Re 30,738) used for the production of hydrocarbons may be modeled in a similar fashion to the fringe field applicator 10 of FIG. 1 and includes spaced outer parallel conductors 16, 18 and a central parallel conductor 17 therebetween. The conductors 16, 17, 18 may be, for example, rows of pipes. Excitation by an RF source 19, as between the central conductor 17 and the outer conductors 16, 18, establishes a fairly well confined electric field. Because heating rates are very high near the conductors, the regions near the conductors will quickly dry out and function like the air gaps in the fringe field applicator. Thus the "tri-plate" applicator 15 of FIG. 2 performs in a manner similar to the fringe field applicator 10 of FIG. 1 and has the additional disadvantage of having to be inserted into the earth. In addition, for shallow treatments, substantial fringing will occur from the bottom of the "tri-plate" conductors unless they are more closely spaced than the depth of treatment.